This invention relates to an apparatus for removing tensile residual stress remaining in welded joint parts of unit pipes and the like.
More particularly, this invention relates to an apparatus for removing, by mechanical means, tensile residual stress due to welding heat remaining in the welded parts when ends of pipes are mutually joined by welding in the circumferential direction.
As is known, a great quantity of piping is used in various kinds of plants and transporting systems. For reasons of limitation and convenience in processes such as manufacturing pipes and transporting the same to their installation sites, pipes are ordinarily produced as unit pipes of specific lengths and then, at the installation site, are joined together to design lengths to install the desired piping.
In such cases, for purposes such as mutually joining a pair of unit pipes and securing a pair of pipes to a support bracket of a plant frame, a welded joint is considered to be a positive and reliable means and is thus used.
When two pipes are joined end-to-end by butt welding around their circumference, for example, as described more fully hereinafter, tensile residual stress is caused to develop considerably in the welded end parts of the pipes in the vicinity of their inner wall surface by the effect of the heat applied in the welding process. This residual stress is apt to become a cause of stress corrosion cracking of the piping during later use. Another objectionable feature of ordinary welding of this character is that it leaves an annular welding back bead which protrudes inward from the inner wall surface of the pipes and gives rise to an increase in the turbulent flow resistance to the flow of the fluid being transferred through the piping and to pressure loss.
A number of measures have been and are being employed to eliminate these objectionable features or to minimize their effects. In one such method, the parts of the pipes to be welded are placed in a cooled state prior to welding and then the welding is carried out, thereby to suppress the development of tensile stress. Another method comprises cooling the pipes after welding and heating the welded parts, thereby to remove tensile residual stress.
In the practice of such a known method, however, heat control is very troublesome, and the operation is extremely complicated. Furthermore, these methods are also accompanied by problems such as high due to cost items such as labor and the necessity of using large-scale equipment.